In the standardization of 3GPP (Third Generation Partnership Project) LTE (Long Term Evolution) an orthogonal single carrier transmission scheme with frequency multiplexing of users is selected for the uplink. The uplink transmission scheme proposed for LTE is known as DFT-S-OFDMA and the basic principle is depicted in FIG. 1.
At step 100, a size N DFT (Discrete Fourier Transform) is first applied to a block of N modulation symbols Ndata. This transforms the modulation symbols to the frequency domain. Next, at steps 110-120, spectrum shaping of the thus transformed symbols Ndata is applied in the frequency domain. The first step at 110 involves the bandwidth expansion of the DFT-transformed modulation symbols through block repetition into a larger number of symbols Nused, while the second step at 120 comprises the filtering of the expanded symbols in the frequency domain.
After spectrum shaping, mapping is done to the IFFT (Inverse Fast Fourier Transform) inputs Nused at step 130. This mapping can be performed in several different ways. Two different mappings, often referred to as localized and distributed mappings, have been proposed for LTE. In case of localized mapping, the mapping is done to consecutive IFFT inputs and in case of distributed mapping the mapping is done to equally spaced IFFT inputs. Thereafter, at step 140 the mapped modulation symbols Nifft are IFFT-transformed forming a sequential data stream. Finally, at step 150, a so called CP (Cyclic Prefix) is attached to the sequential data stream in order to avoid ISI (Inter Symbol Interference) and ICI (Inter Carrier Interference) at the receiver. The transmitted signal is a low-PAR (Peak-to-Average Power Ratio) “single-carrier” signal despite the apparent “multi-carrier” structure.
The difference between the traditional OFDMA-structure commonly used in wireless communication networks and DFT-S-OFDMA is that in traditional OFDMA the data symbols are directly mapped onto an arbitrary set of sub-carriers while in DFT-S-OFDMA the data symbols are first transformed by a DFT and then mapped to either a consecutive or an equally spaced set of sub-carriers.
In case of localized mapping however, the mapping onto consecutive sub-carriers in DFT-S-OFDMA leads to several problems. If radio resources for a user in such a wireless communication network are scheduled in the middle of the frequency band then the remaining transmission resource becomes fragmented into two parts. The next user to be scheduled resources may then only use the scheduled resources in one of the remaining fragments as a consequence of the single carrier restriction. This limits the achievable bit rate of that user.
Another area which may result in uplink single carrier frequency fragmentation is the application of DFT-S-OFDMA to frequency hopping. Even if consecutive frequency allocations may be allocated to different UEs in one time interval problems will arise when users hop around in frequency. This becomes a significant problem in case all users are assigned frequency allocations of different sizes.
Also, when it comes to inter cell interference coordination solutions for the uplink resource fragmentation may become a problem. If, for example, it is desired to make it possible for cell edge users in different cells to communicate on orthogonal uplink resources, then a situation may arise where the cell edge user may be allocated transmission resources in the middle of the frequency band which will lead to a fragmented resource in that cell.
It is an object of the current invention to resolve the shortcomings of the currently proposed LTE single carrier solution.